Electrical transmission system



March 17, 1942. w HARDING m- 2,276,652

ELECTRICAL mzgnsugtsszon sys'ram Filed May 4, 1938 6 Sheets-Sheet 1 A 25 43 26 27 c 28 T P P 3 40 INVENTORS MLLIAMGHARDINQ N ROBERT HNIsBET March 17, 1942. w HARDlNG ETAL 2,276,652

ELECTRICAL TRANSMIS S ION SYSTEM Filed May 4, 1958 6 Sheets-Sheet 2 Fig. 3.

C 32 mvEmToRs WILL/HM Gf/nnnufi 1 figgERT H.NISBET 1 1 a r ATTORNEY March 17, 1942. w. G. HARDING ETAL 2,276,652

I ELECTRICAL TRNSMISSiON SYSTEM Filed May 4, 1938 6 gnaw-ghee; s

E INVENTOR5 WILL/AM QHHRDING fiOBERTH, NISBET v v B) I at ATTORNEY.

March 17, 1942. w HARDmG ETAL' 2,276,652

ELECTRICAL TRANSMI S S ION SYSTEM Filed-May 4, 1938 6 Sheets-Sheet 4 INVENTORS MLLIHM G. h'nanw fi ROBERT/7f NISBET March 17, 1942. w. G. HARDING E.TA L

ELECTRICAL TRANSMISSION SYSTEM 7 6 Sheets-Sheet 5 Filed May 4, 1938 INVENTORQ MLL/HM 6. finnmlvfi /10BERT H'N/SBET ATTORNEY i March-17, 1942. W. G. HARDING HAL 2,276,652

ELECTRICAL TRANSMIS S ION SYSTEM Filed May 4, 1939 s Sheets-Sheet s AMPILIFIER AMPLIFIER GENERATOR I WNVENTORS Mzunlvdllnfiuwe'i fiaBe'nr/i N/sBEr I ATTORNEY.

Patented Mar. 17, 1942 ELECTRICAL TRANSMISSION SYSTEM William George Harding, Whitton, and Robert Hayes Nlsbet, Osterley, England, assignors to Sperry Gyroscope Company, Inc., Brooklyn, N. Y., a corporation of New York Application May 4, 1938, Serial No. 206,0il8 In Great Britain May 7, 1937 4 Claims. (Cl. 172-239) thereof will now be described with reference to the accompanying drawings, in which---- Fig. 1 is a circuit diagram of a system according to the invention employing a half-wave rectiflcation circuit;

Fig. 2 is a circuit diagram of a similar system,

but wherein full-wave rectification is employed;

Fig. 3 is a circuit diagram of a three-line stepby-step transmission system? Fig. 4 illustrates one arrangement of a pair of grid-controlled rectifiers for use in the circuit of Fig. 3;

Fig. 5 illustrates an alternative arrangement to that of Fig. 4; i

Fig. 6 illustrates a transmitter suitable for use in the circuit of Fig. 4 or Fig. 5;

Fig. 7 is a circuit diagram of a three-line stepby-step system similar to that of Fig. 3 but wherein the delta arrangement of the rectifiers is replaced by a "star arrangement thereof;

Fig. 8 illustrates a dynamo-generator 4-line system for amplifying D. C. step-by-step signals,

embodying the invention;

Fig. 9 illustrates a system similar to that of Fig. 8 but providing a 3-1ine system;

Fig. 10 is a diagram of an alternative arrangement to that of Fig. 9, showing the several generators combined into one;

Fig. 11 is a circuit diagram of an electrical 3- line Selsyn transmission with follow-up control.

Referring first to Fig. 1, reference numerals l 2. 3 denote grid-controlled gas-discharge rectifier tubes, the cathodes 4, 5, 6 of which are connected together and to terminal I of the secondary winding of a transformer 8 the primary winding of which is connected to a source of alternating current. The other terminal 9 of transformer 8 is connected to the anodes III, II. l2 of the tubes l, 2, 3 through impedances l3, l4, l5 respectively.

The cathodes 4, 5, G of the grid-controlled rectiflers are also connected to a tapping-point IS on a resistance I! connected across a source of direct current so that the cathodes are positive with respect to the leg iii of the D. C. supply. The grids of the three tubes are connected re-' spectively to tapping points I9, 20, 2| on resistances 22, 23, 24, each of which has one end connected to the negative leg l8 of the D. C. supply, while their other ends are connected respectively to common side contact fingers of the contact gaps 25, 26, 21. The contact fingers of the other side of the contact gaps are all connected to the positive leg 28 of the D. C. supply.

If the contact gap 25 is open, the grid of tube I is at the same potential as the negative leg of the D. C. supply, whereas the cathode is connected to point I6 which is more positive than this. The tapping-point I6 is so selected that the grid of tube I is sufliciently negative in these circumstances with respect to the cathode 4 to prevent current from passing. When the contact gap 25 is closed, the resistance 22 is connected across the D. C. supply, and the tapping-point I9 is at once made positive with respect to the negative leg l8. The position of the tapping point i9 is so chosen that, when the contact gap 25 is closed, the potential of the point [9. and therefore that of the grid of tube l, becomes positive with respect to the cathode 4, so that the tube becomes conductive. Rectified current then flows from terminal 9 of transformer 8 through impedance l3, thus producing a difference of potential, having both D. C. and A. C. components, across impedance l3. This difference of potential may act as a source of supply to apparatus connected across impedance l3.

The contactor gaps 25, 26, 21, having pairs of contact fingers denoted by the letters A, B, C, represent the contacts of a four-line step-by-step transmitter of well-known type for transmitting movements such as the relative turning of the follow-up element of a gyro-compass and a ship. The transmitter. is connected so as to be operated'by the turning movement in question, and, when so operated, it closes the contact gaps 25, 26, 21 in succession in the following recurring sequence for continuous motion in one sense, i. e., the contact fingers are closed in succession in the following order: A, AB, B, BC, C, CA. It

follows that the tubes I, 2, 3 become energised in the corresponding sequence and therefore that voltages are produced across the impedances l3,

l4, IS in this sequence.

Connections are taken from the anodes "I, H, l2 tothe star-connected windings A, B, C of repeaters, one of which is shown at 29; the starpoints of the repeaters are connected to the common point 9' of the impedances l3, l4, 15. Wind-- ings A, B, C are therefore subjected to whatever voltages are developed across impedances I3, l4, [8 so that, under the conditions assumed, they are energised in the recurring sequence: A, A'B', B, BC', C, C'A', just as if they had been connected directly to the correspondingly lettered transmitter contact gaps 25, 23, 21. The energisation of the windings therefore produces a magnetic field in the repeater 29 that rotates in steps thus causing the armature, which may be polarised or unpolarised, to follow this field by rotating in a step-by-step manner.

With the system of the present invention it is possible to operate a large number of repeaters such as 29, since the tubes I, 2, 3 can handle quite large currents. In spite of this, the resistances 22, 23, 24 can be very high, so that not more than a few milliamps need be handled by the transmitter contacts 25, 23, 21; sparking and buming of the latter are therefore prevented.

Since the voltages across impedances I3, l4, II have A. C. and D. C. components, the cur rents flowing in the windings A, B, C, of the repeaters 29, etc., will have A. C. and D. C. com- .ponents. If desired, the alternating current components may be filtered out by the use of fliter circuits. Since the windings A, B, C are inductive it may be found sufficient for this purpose to by-pass the windings by condensers, or, what amounts to the same thing, to include condensers as circuit elements in parallel with the impedances I3, l4, l5. However, a certain amount of A. C. in the windings A, B, C' is advantageous. Thus, when' the supply of current ceases in winding A owing to the breaking of the gap 23, there will be current in either winding B or C; if this current has A. C. components these will affect the core of winding A and help to demagnetise it more effectively than would occur by the mere cessation of current in winding A. The operation of the repeater 29 is therefore more definite and accurate at high speeds with A. C. and D. C. than with D. C. alone.

If desired, therefore, all filtering may be dispensed with, in which case the impedances l3,

l4, IS in parallel with the windings A, B, C of the repeaters 29, etc. may be omitted from the circuit.

Full-wave rectification circuits may be employed instead of the half-wave rectification circuit shown in Fig. 1. In Fig. 2 there is shown an arrangement in which the circuits for tube l of Fig. l are replaced by full-wave rectification circuits employing two tubes l and I". As shown, two separate D. C. supplies I8, 23', and I3", 29" are required for supplying the grid voltages for tubes l' and I respectively; these may be obtained from additional secondary windings on the transformer 8 by the aid of rectifiers. Similarly the contact gaps and 25" (which need 'to be operated simultaneously to give fullwave rectification) must be separate: the system therefore requires 6 transmitter contacts operating in pairs. This result may be obtained by usv ing two transmitters arranged to operate corresponding contacts simultaneously.

A somewhat similar method of amplification may be employed for the three-line step-by-step transmission system in which the repeater coils are energised in either direction, 1. e., either positively or negatively. The principle of this method is shown inFig. 3 in which the repeater 30 is of the standard type for this system of transmission, the stator having three coils connected in delta formation. The transmitter 3| is represented as including three arms containing rectifiers also connected in delta formation the vertices being the points 32, 33, 34. Each arm of the transmitter contains two branches with rectifiers, such as A1 and A2, which are connected between the points 33, 34.

The rectifier A1 is adapted, when excited by means described hereinafter, to drive current from 33 to 34; similarly the rectifier A2 is adapted, when correctly excited, to drive current from 34 to 33. Rectifiers BlB2CiC2 are similarly adapted to drive current in one direction, the directions of the currents produced by AlBlCl being clockwise around the transmitter and those produced by AzBaCz being anti-clockwise.

In Fig. 4 A1 and A2 are shown as grid-controlled rectifier tubes, the cathodes of which are supplied from winding 31 of transformer 33 and winding 39 of transformer 40 respectively. Both rectifiers are supplied from the centre-tapped winding 4|, 42 of transformer 40, the primary winding 43 of which is connected to an A. C. sup ply. The winding 4| is connected between point 33 and the anode of A1 while the winding 42 is connected between point 33 and the cathode of A2. Point 34 is connected to the cathode of A1 and to the anode of A2. There are thus two branch paths between 33 and 34 each containing a secondary winding of transformer 40 and a rectifier tube. The branch containing A1 is therefore adapted to pass current from 33 to 34 or else to interrupt such current according to the manner in which the grid of A1 is excited, while the branch containing A2 is adapted to pass current from 34 to 33 or else to interrupt such cur rent according to the manner in which the grid of A: is excited. Fig. 4 thus illustrates how the arrangements illustrated schematically in Fig. 3 may be practically embodied.

,The grid circuits for controlling the rectifiers are also shown in Fig. 4. Transformer 38 has a tapped secondary winding 45, the tapping point being connected to the centre-tap of the cathode winding 31 of tube A1. Winding 45 is connected to the grid of A1 through an impedance 46 so as normally to apply agrid voltage in opposite phase to the anode voltage, whereby the tube A1 is normally non-conductive. However, when the transmitter contact 41 is closed the grid of A1 is directly connected to the end of transformer winding 44 thus giving the grid an opposite voltage so that the tube becomes conducting.

Fig. 5 shows an alternative arrangement for the grid control of rectifier tubes arranged in a circuit similar to that of Fig. 4. The grid of tube A1 is connected to the cathode supply winding 31 of transformer 33 through the secondary windings 48 and 45 of transformers 49 and 38 respectively each secondary winding being shunted by a resistance to keep the grid-cathode-circuit impedance low. Transformer winding 45 supplies a grid voltage in opposite phase to the anode voltage so as to extinguish the tube when no voltage is applied by transformer 49.

The primary winding 59 of transformer 49 is in series with the secondary winding of transformer 5| and with transmitter contact 41' so that, when this contact is closed an A. C. voltage is introduced into the grid circuit of tube A1 by the secondary winding 48. This voltage is in phase with the anode voltage and is of sufficient magnitude to overcome'the voltage applied by transformer 38 and render the tube conducting.

It is obvious that the tube A1 would be extinguished not merely if transformer 49 is applying no voltage to the grid circuit but also if it is applying a voltage in opposite phase to the anode voltage. Consequently a variation of the circuit of Fig. may be employed in which transformer 49 sometimes applies an in-phase voltage, some times no voltage and sometimes an out-of-phase voltage to the grid of A1, the anode voltage of A1 being taken as reference. The six tubes A1A2B1B2C1C2 can therefore be made conductive in any desired sequence by closing the grid contacts in that order.

In order to obtain the sequence of steps of energisation for the windings of the repeater 30 normal for this repeater during turning of the transmitter in one direction, the tubes must be operated in the sequence:

A1, AiBz, B2, BzCi, C1, ClAZ, A2, A231, Bi, B1C2,'

C2, C2A1. This results in current flowing in any one coil in repeater 30 for periods of five steps in each direction with one intermediate step of zero current between periods.

In the arrangement of Fig. 4 six separate transmitter contact pairs are required, and there are no two contacts that are in direct electrical connection. Twelve lines are therefore required from the transmitter. Fig. 6 illustrates ,a transmitter contact arrangement in which the number of contacts to be operated is considerably reduced. In this figure the transmitter consists of three similar cams 52, 53, 54 angularly spaced. apart 120 on a single shaft.. Rotation of the.

cam shaft causes cam 52 to oscillate a finger carrying a contact 55 so as to cause 55 to contact alternately with two contact stops 56, 51, with intermediate periods during which 55 contacts with neither of stops 56, 51. Contact stops 5t, 5? are connected to opposite ends of the sec= ondary winding $8 of transformer 59, the primary winding of which is connected to the A. C. supply. The primary windings of two transformers til, 655 are connected in parallel between contact finger and the centre-point or winding 58. It follows that the transformers 59, til are energised simultaneously by voltages in phase with the A. C. supply or in phase-opposition thereto according as finger as makes-contact stop tit or 5']. Transformer till controls tube A1, acting in the same manner as transformer (it Fig. 5 while transformer (it controls tube A2.

Each of the earns 53, 56 operates a finger in the same manner as does cam 52 in order to control tubes B1B2-C1C2 of Fig. 3.

The contacts and 5?] produce three conditions of energization of the of tubes i1 and A2. In the first con ition, contacts are closed, so that transformers 69 and 69 are energized from the upper half or secondary windin a transmitter cycle of twelve steps, each tube should be conducting for three steps, which is ensured if-the pair of contacts 55 and 56 make for three steps and the pair of contacts 65 and 61 for three steps, with intervening gaps of three steps on each side in which neither pair of contacts is made.

Since all the voltages in Fig. 4 are in-phase or in direct phase-opposition to each other it is evident that the transformers 38 and 40 need not be independent transformers but can be replaced by a single transformer with separate secondary windings.

anode voltages of all the tubes are in-phase or fingers operated by the cams 53, 54 for controlling the remaining tubes can therefore beelectrlcally connected to stops 58, 51. The transmitter therefore requires only 5 electrical connections, one for each of the three fingers and The conditions of operation required are that, '55

two for the two sets of three contact stops.

Fig. '7 shows an alternative circuit arrangement equivalent to that of Fig. 3, the three pairs of rectiflers such as A1A: being con'nected'in star formation instead of in delta formation. The circuit details of Figs. 4, 5 or 6 may be used in this arrangement, except that the sequence of energisation must be somewhat different from that suitable for Fig. 3. r The system of Fig. 7 requires that curren must flow from 3! to 34 for five transmission steps and from 34 to 3| for five transmission steps, and that there must be, between each group of flve steps, an interval of one transmission step during which no current flows in the branch between 3! and 34. in order to obtain this result, A1 must be energised positively (to make it conductive) and A3 negatively by a signal of five steps duration; this must be followed by an'interval of one step during which no signal is applied to A1 or A2; there must then follow an interval of five steps during which A11 is energised positively and A1 negatively; and, finally, to complete the cycle, there must be another interval of one step which neither A1 nor A2 is energised. The correct cycle of operation can be provided by a transmitter ilar in all respects to shown through which each on any one contact st Commute-tor tra with brushes or re purpose. In particu mitter of British No.

contrast with th described above, in are employed there systems employing tiers. In these syste motor drives one or currents from the the generators generate voltages propel the signals from the master transmitter.

It has previously been proposed to employ 22 Furthermore, it is clear that, as the the excitation of a generator whose output was led to receivers. In these systems, however, the transmitter signals were direct currents as long as the transmitter was stationary and the generator generated corresponding D. C.-' voltages which were applied to synchronous motors acting as the receivers. The receivers had continuously wound stators, and the currents supplied by the generator excited a constant magnetic field, the direction of whose axis was determined by the position of the transmitter.

In such systems, the position of rest of the receiver is defined entirely by the relative magnitudes of the currents in the windings, and not by any permanent structure of the magnetic field such as slots or salient poles. The transmission system in which these receivers were used was usually provided with a transmitter designed to change very gradually with rotation the output currents that it provided, so that the receivers turned continuously and smoothly. However, to effect this, these systems had the disadvantage that the positions of rest of the receivers were not sharply defined so that the indications were appreciably altered by friction or by other disturbing torques.

We refer to such systems as graduated transmission systems since the magnetic structure of the receiver stator is such as to permit graduated and continuous rotation and we refer to them as graduated D. C. systems because the currents supplied to the receivers are direct currents when the transmitters are not rotating.

step-by step transmission, in which at every step full current is switched into'or out of a winding of a receiver having sharply defined or salient poles, gives very much sharper definition to the position taken up by the receiver motor. Each characteristic position is therefore reproduced accurately and with considerable torque, and for many purposes this accuracy is more important than smoothness of motion.

In circumstances where smoothness of motion is of importance A. C. transmission possesses advantages as compared with graduated D. C. transmission. Both are graduated in their action but the signals for D. C. transmission are provided by a potentiometer or by brushes movable on the commutator of a rotating electro- -dynamic machine, and there are always present contact or commutation troubles which are much more serious than any experienced with the slip rings of A. C. transmitters or receivers. In addition, the circuits empioyed are inductive, so that lagsin operation are obtained with D. C. transmission. Finally, residual magnetism causes quite appreciable errors in a graduated D. C. repeater, whereas the alternating currents used in A. C. transmission are completely free from this source of error.

The previously proposed graduated D. C. transmission system cannot therefore entirely replace either the D. C. step-bystep transmission sys tems or the graduated A. C. systems. Both of the latter two systems are in extensive use and it is frequently necessary to enlarge an existing installation of either type so that a larger number of receivers can be driven than was initially contemplated.

According to the present invention, we provide means for supplying a large number of receivers (or a heavily loaded receiver) either of the stepby-step type or of A. C. type with currents ,for positioning the same, said means comprising a 2,276,652 transmitter, the signals from which controlled constantly running generator whose output is controlled by excitation currents supplied or controlled by a transmitte Fig. 8 illustrates an mbodiment of the invention as applied to a D. C. step-by-step transmis-' sion system. In this figure the three D. C. generators 52, 63, 84 on a common shaft are driven by a motor 85 which has its armature and field winding 66 both supplied from a D. C. source. The field windings 61,, 68, 69 of the generators 62, 63, 6'4 are supplied from the same D. C. source, the negative leg being connected to all three windings but the positive leg being connected only through the transmitter 10. three contact segments ll, 72, 13 on which rest .two angularly spaced contact brushes I4, 15,

E. M. F.s are proportional to the currents pro duced in the field windings by the transmitter 10 and they may therefore be used to produce, in external load circuits connected to them, amplified versions of these currents. For this purpose the negative brushes of the three generators are connected together and connected to the negative line 15 of the outgoing transmission system, while the three positivebrushes are each connected to one of the lines I1, I8, 19. These four lines leading to repeaters therefore provide fourline step-by-step transmission of the same wellknown type as that provided by transmitter 10. Fig. 9 illustrates a similar method of amplification for step-by-step transmission of the 3-line D. C. type. The transmitter 10' has three contact segments H, 12, 13', as in the construction of Fig. 8, but these segmenm are energised from a rotatable carriage having two pairs of brushes or rollers M, l5 and 14", 15". The contacts M, 15' are angularly spaced and are connected together to the negative leg of the D. C. supply, while the contacts 1d", '85" are connected to the positive leg. When the transmitter rotates, the three segments ll, 52', 13 are therefore each energised alternately positively and negatively from the D. C. supply. The field windings 61, 68', 69' of generators 52, t3, '3 are connected in star formation between the lines running from the three segments H, 122', it and are thus energised alternately positively and negatively. Corresponding E. M. F.s are generated in the armature windings which are also connected in star formation across the lines Ti, 18', 19' leading to the repeaters. The generators 52', S3, 64 generate voltages proportional to those applied to their field windings and therefore produce voltages across the lines W, l8, l9, corresponding to those developed on the contact segments ll, 52', H3; in other words amplified transmission signals are sent out along the lines ii, iii, i9.

Fig. 10 illustrates a variation of the arrangement shown in Fig. 9 in which the three generators 62', G3, 66' of that figure are replaced by a single generator 86 having a fully wound stator the windings of which are closed and resemble those of a three-phase induction motor. The three transmitter segments ll, 12', 73' are connected to equidistant tapping points on the stator winding so that the three segments 51'', 88", 89" of the windings correspond to the field windings The latter has 67', 08', 69' of Fig. 9'except that they are connected in delta formation instead of in star formation. The rotor has a commutator connected to a similar closed winding and three brushes 80, 8|, 82, spaced 120 apart rest onthe commutator in such positions that the voltages between the pairs of brushes are proportional to the excitation currents in the windings 61", 60",

The brushes 30, 8|, 82 are connected to the output transmission lines 11, I9, I9 through compensating windings 03, 84, 85 on the stator. These windings are intended to neutralize the field set up on the rotor by the armature currents drawn from the brushes 80, 8|, 82, respectively, to supply the lines II, III, I9, so that the fields produced by the windings GI", 68'', 09" are not distorted thereby.

A generator of the type shown at 86 is also suitable for use as an amplifier for .Selsyn A. C. transmission signals. In such a system the transmitter may be a "Selsyn transmitter of the Fig. 11 shows a system of transmission in which I electrical amplification of signals is associatedwith a follow-up system. The element 99 is the one from which transmission is to be effected, but instead of gearing a transmitter to this, a follow-up element I is provided, and a twopart controller IOI, I02. The latter is preferably of the inductive or balanced variable-gap transformer type described-in U. S. Patent No. 1,959,804 to B. A. Wittkuhns et al., dated May 22, 1934, in which the member IIII is an iron armature lying symmetrically in the field of a three-legged transformer core I02. The centre leg of this core carries the primary winding which is energised from a source of alternating current and two secondary windings connected in series opposition are disposed on the outer legs. The output of these two secondary windings in series is suitable for the control of a up or repeat-back connection from servo-motor I04 to the controller IOI, I02 for this servomotor, as will now be explained. I

.If member 99 moves, the two-partcontroller applies an input to the amplifier I03 thus causing motor I04 to run in a direction corresponding to that in which member 99 has moved. The carriage I05 rotates causing changes in the voltages applied to lines I I2, I I3, I M. This in turn causes changes in the voltages applied by the generator II5 to lines H6, H1, H8 so that all the repeaters turn by corresponding amounts. In particular, repeater 9 turns the rotor I23 driving the follow-up member I00 to follow member 99. The servo-motor I04 runs until the follow-up member I00 overtakes the member 99 at which point the output of the two-part controller IOI, I02, to the amplifier I03 falls to zero, and the motor I04 stops. It is to be remarked that in this system all errors in amplification due for example to the voltages picked off the potentiometer III being non-proportional to the angle of turn of brushes I06, I01, or due to the E. M. F.s generated by generator II5 being non-proportional to the currents in the excitation windings, are automatically neutralized by this system. The motor I04 runs until repeater H9 is correctly matched with member 99; the transmission signals along lines H0, H1, H0 are therefore correct, since the repeater I I9 is positioned by them, the motor I04 being controlled to run so as to take out inherent errors. It follows that all the other repeaters actuated by the transmission signals in lines I I6, III, II8 also become correct.

It is well known that gyro-compasses require a follow-up member from which transmission may be effected. When the system of Fig. 11 is used for transmission from a gyro-compass, the

member 99 may be taken as representing the sen- .sitive element of the compass.

The member I00 must therefore be a member that follows up the sensitive element so as to control servo-motor I04 to cause the correct transmission signals to be sent out. The follow-up member I00 may be additional to any follow-up member already present in the compass: alternatively it may be the sole follow-up member and serve all the purservo-motor, as is explained in U. S. Patent 2,054,945 to R. H. Nisbet, dated September 22, 1936. The output is passed to the amplifier I03 which controls the servo-motor I04 to run in one direction or the other according to the sense of the displacement of member 99 relative to member I00, e. g., in the manner explained in U. S. Patents 1,959,804 and 2,054,945 above-mentioned. On the shaft of-the servo-motor I04 is fixed a carriage I05 carrying two contact rollers or brushes I06, I01 connected to slip rings I08, I09 by which current is led in from an A. C. supply H0. The rollers or brushes I06, I01 make contact with the potentiometer winding III, from which connections H2, H3, H4 are taken at three fixed equally spaced tapping points. These connections lead to an amplifying generator II5 similar to that shown in Fig. 10. The output of generator ll5is taken by the three lines H8, I I1, I I8, to Selsyn repeaters, one of which is shown at H9. The primary windings of the Selsyns, e. g., winding I20, are supplied with single-phase currents from a generator I2I on the same shaft as generator I I5 and driven from the same motor poses of the so-called phantom element which acts as a suspension for the sensitive element in a well-known type of compass. In either case, the following member I00 follows the sensitive element 99 of the gyro-compass and therefore all the repeaters are subject to the north steaming error of the gyro-compass. In order that the repeaters should be corrected, a corrector may be used at the gyro-compass so that the part IOI of the two part controller is displaced by the correct amount relative to the sensitive element to correct the error, or so that the part I02 is displaced relatively to the part I00 by this amount. Correctors for automatically varying correction as the ship changes course are shown in U. S. Patents 2,110,766 to W. G. Harding, dated March 8, 1938, and 2,128,559 to W. G. Harding et al., dated August 30, 1938. An alternative method of introducing corrections is to do so in the supply lines to the rotor I23 of Selsyn" H9, in which case the corrector mechanism could be operated by a repeater motor remote from the compass, e. g., one supplied from the lines H6, H1. H8.

For this purpose, the connections supplying the Selsyn secondary windings I23 would be taken, not from the lines H8, H1, H8, but from the secondary windings of a Selsyn" diiferential (not shown), whose primary windings were supplied from the lines Hi6, iii, M8. The corrector would produce relative angular movement of the stator and rotor of this fielsyn diiferential and thus introduce the required correction.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be periormed, we declare that what we claim is:

1. In a positional control system having a controlling element and a controlled element, intermediate control means comprising a plurality of grid controlled gas discharge rectifier tubes having grid and plate circuits, an A. 0. supply for said plate circuits, an independently adjustable source of potential connected to each of said grid circuits, contact means arranged to be actuated in definite sequence by said controlling element and connected to adjust said grid circuit potentials in a manner causing currents to flow in similar sequence in the respective plate circuits of said tubes, and a motor havingv a stator and a rotor, said rotor being connected to position said controlled element and one of said motor members having windings respectively connected to the plate circuits of said tubes to be energized by currents therein, the position assumed by said rotor being determined by the energized winding or windings.

2. A positional control system as defined in claim 1, wherein said grid controlled rectifier tubes are arranged in pairs in parallel, the two tubes of a pair being adapted to pass current in opposite directions.

3. A positional control system as defined in claim 1, wherein said grid controlled rectifier tubes are arranged in pairs in parallel, the two tubes oi a pair being adapted to pass current in opposite directions, means for biasing both tubes of a pair so that the same are normally nonconducting, said contact means being so actuated that opposite control potentials are applied to the grids of the two tubes, whereby only one tube of a pair is rendered conductive at a time.

4. In a D. C. step by step transmission system having a controlling element, a controlled element and a multi-pole motor for positioning said controlled element at discrete positions corresponding respectively to the energized pole or poles, a source of alternating current, a plurality of rectifier tubes having control grids and having their outputs respectively connected to energize said motor poles, circuit means for energizing the plate circuit of said tubes from said A. C. source, a source of direct current, a transmitter adapted to apply potentials derived from said D. C. source to the grids of said tubes, in sequence, individually and in pairs, for energizing the respectively connected poles of said motor.

WILLIAM GEORGE HARDING. ROBERT HAYES NISBE'I. 

